Organic light emitting devices have been known for approximately two decades. OLEDs work on certain general principles. An OLED is typically a laminate formed on a substrate such as soda-lime glass or silicon. A light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between a cathode and an anode. The semiconductor layers may be hole-injecting or electron-injecting layers. The light-emitting layer may be selected from any of a multitude of fluorescent organic solids. The light-emitting layer may consist of multiple sublayers or a single blended layer.
When a potential difference is applied across the device, negatively charged electrons move from the cathode to the electron-injecting layer and finally into the layer(s) of organic material. At the same time positive charges, typically referred to as holes, move from the anode to the hole-injecting layer and finally into the same organic light-emitting layer(s). When the positive and negative charges meet in the layer(s) of organic material, they combine, and produce photons.
The wave length--and consequently the color--of the photons depends on the material properties of the organic material in which the photons are generated. The color of light emitted from the OLED can be controlled by the selection of the organic material, or by the selection of dopants, or by other techniques known in the art. Different colored light may be generated by mixing the emitted light from different OLEDs. For example, white light is produced by mixing blue, red, and green light simultaneously.
In a typical OLED, either the anode or the cathode is transparent in order to allow the emitted light to pass through to the viewer. The cathode is typically constructed of a low work function material. The holes are typically injected from the anode, a high work function material, into the organic material via a hole transport layer.
Typically, OLEDs operate with a DC bias of 2 to 30 volts. The OLED brightness may be controlled by adjusting the voltage or current supplied to the anode and cathode. The relative amount of light generated is commonly referred to as the "gray level." OLEDs typically work best when operated in a current mode. The light output is more stable for constant current drive than for a constant voltage drive. This is in contrast to many other display technologies, which are normally operated in a voltage mode. As a result, an active matrix display using OLED technology, requires a specific pixel architecture to provide for a current mode of operation.
In a typical matrix-addressed OLED device, numerous OLEDs are formed on a single substrate and arranged in groups in a regular grid pattern. Several OLED groups forming a column of the grid may share a common cathode, or cathode line. Several OLED groups forming a row of the grid may share a common anode, or anode line. The individual OLEDs in a given group emit light when their cathode line and anode line are activated at the same time. A group of OLEDs within the matrix may form one pixel in a display, with each OLED usually serving as one subpixel or pixel cell.
OLEDs have a number of beneficial characteristics. These include: a low activation voltage (about 5 volts); fast response when formed with a thin light-emitting layer; high brightness in proportion to the injected electric current; high visibility due to self-emission; superior impact resistance; and ease of handling of the solid state devices in which they are used. OLEDs, have practical application in television, graphic display systems, and digital printing.
Although substantial progress has been made in the development of OLEDs to date, additional challenges remain. For example, OLEDs continue to face a general series of problems associated with their long-term stability. In particular, during operation the layers of organic film may undergo recrystalization or other structural changes that adversely affect the emissive properties of the device.
Exposure to air and moisture presents unique problems with respect to OLEDs. Exposing a conventional OLED to the atmosphere shortens its life. The organic material in the light-emitting layer(s) reacts with water vapor, oxygen, etc. Lifetimes of 5,000 to 35,000 hours have been obtained for evaporated films and greater than 5,000 hours for polymers. However, these values are typically reported for room temperature operation in the absence of water vapor and oxygen. Lifetimes associated with operations outside these conditions are typically much shorter.
The low work function cathode is susceptible to oxidation by either water vapor or oxygen. Electroluminescence from oxidized areas is typically lower than other areas. The anode may also be affected by oxidation. The penetration of oxygen and moisture into the interior of the OLED may result in the formation of metal oxide impurities at the metal-organic material interface. These metal oxide impurities may cause separation of the cathode or anode from the organic material. Dark, non-emitting spots may appear at the areas of separation due to a lack of current flow. Cathode materials such as Mg-Ag or Al-Li are especially susceptible to oxidation.
To obtain a practical, useable OLED, it is necessary to protect the device, so that water, oxygen, etc., do not infiltrate the light-emitting layer or oxidize the electrodes. Methods commonly employed for protecting or sealing inorganic electroluminescent devices are typically not effective for sealing OLEDs. For example, in the "silicon oil method" of sealing inorganic electroluminescent devices, the silicon oil can infiltrate the light-emitting layer of an OLED, the electrodes, and any hole-injecting or electron-injecting layers. This may alter the organic light-emitting layer, reducing or eliminating its light emission properties. Likewise, resin coatings that have been used to protect inorganic EL devices are not suited for OLEDs. The solvent used in the resin coating solution tends to infiltrate the light-emitting layer of the OLED, degrading the light emission properties of the device.
Protective films may be used to seal OLEDs. For example, an electrically insulating polymer may be deposited on an outer surface of the OLED. Evaporated metal films are also used to seal OLEDs in a similar manner. Evaporated metal and polymer films are both susceptible to pinholes. To avoid pinholes these films must be relatively thick and hence result in poor light transmission. Accordingly, there remains a need for a method of sealing an OLED which does not degrade light emissions from the device.
Edge shorting between the cathode and anode layers is another problem affecting most conventional OLED devices. Edge shorting reduces the illumination potential of the display. Edge shorting is the channeling of light within the organic layers. As a result of the channeling, light is not directed toward the viewer. Also, when light is emitted at all forward angles, i.e., in a Lambertian manner, it may activate neighboring OLEDs reducing contrast or color purity. Thus, there is a need to develop a microcavity structure capable of limiting edge shorting and increasing illumination.
A passive OLED matrix pulses light at high pixel brightness in order to achieve moderate overall brightness. An active OLED matrix with sustained application of voltage across the anode and cathode may produce the same brightness with a much lower pixel luminance. However, in order to achieve the same appearance as the passive matrix, the active matrix must be refreshed continuously. As a result, there is a need to provide a method of refreshing an OLED display during the time that the organic layers are exposed to the electrical potential between, the anode and the cathode.
A typical matrix of OLEDs experiences certain problems. As described above, the OLEDs located within the matrix may experience channelling. The channeling of light in one OLED or subpixel may cause the inadvertent activation of neighboring subpixels. Furthermore, the proximity of the subpixels within the matrix can cause a reduction in ambient light contrast across the matrix. The need exists for a matrix design with improved color purity and ambient light contrast.
The present invention meets the needs set forth above, and provides other benefits as well.